Spectrum spreading techniques for use in digital communication networks have been described in many books and papers. A classic publication in this field is Spread Spectrum Communications by M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levitt, Computer Science Press, 11 Taft Court, Rockville, Md. 20850, 1985. Particular kinds of spectrum spreading techniques that have been implemented in digital communication networks in the prior art include "direct-sequence spreading", "frequency hopping", "time hopping", and various hybrid methods that involve combinations of the aforementioned techniques.
Multi-node spread-spectrum communication networks developed in the prior art were generally characterized as code-division multiple-access (CDMA) networks, which utilized "code-division multiplexing" (i.e., a technique in which signals generated by different spreading-code sequences simultaneously occupy the same frequency band). Code-division multiplexing requires that the simultaneously used spreading codes be substantially "mutually orthogonal", so that a receiver with a filter matched to one of the spreading codes rejects signals that have been spread by any of the other spreading codes.
In a typical multi-node spread-spectrum communication network using either a conventional direct-sequence spectrum spreading technique, or a hybrid technique involving, e.g., direct-sequence and frequency-hopped spectrum spreading, only a single spreading code is employed. At regular intervals, the polarity of the spreading code is either inverted (i.e., each 0 is changed to 1, and each 1 is changed to 0) or left unchanged, depending on whether the next bit of information to be transmitted is a 1 or a 0. The resulting signal is an "information-bearing" sequence, which ordinarily would be transmitted using some type of phase-shift keyed (PSK) modulation--usually, binary phase-shift keyed (BPSK) modulation or quaternary phase-shift keyed (QPSK) modulation.
A publication entitled Spread Spectrum Techniques Handbook, Second Edition, March 1979, which was prepared for the National Security Agency by Radian Corporation of Austin, Tex., describes a number of spread-spectrum techniques that had been proposed in the prior art. Of particular interest is a direct-sequence technique described on page 2-21 et seq. of the Spread Spectrum Techniques Handbook, which involved transmitting one bit of information (either a 0 or a 1) by switching between two independent signals that are generated by different spreading codes. Ideally, the spreading codes of the two independent signals should be "almost orthogonal" with respect to each other, so that cross-correlation between the two sequences is very small. In practice, in such early spread-spectrum communication systems, the two independent signals were maximal-length linear recursive sequences (MLLRSs), often called "M-sequences", whose cross-correlations at all possible offsets had been computed and found to be acceptably low. However, this technique of switching between two independent signals did not achieve widespread acceptance, mainly because it required approximately twice the electronic circuitry of a polarity-inversion technique without providing any better performance.
Two recent papers, viz., "Spread-Spectrum Multiple-Access Performance of Orthogonal Codes: Linear Receivers" by P. K. Enge and D. V. Sarwate, (IEEE Transactions on Communications, Vol. COM-35, No. 12, December 1987, pp. 1309-1319), and "Spread-Spectrum Multiple-Access Performance of Orthogonal Codes for Indoor Radio Communications" by K. Pahlavan and M. Chase, (IEEE Transactions on Communications, Vol. 38, No. 5, May 1990, pp. 574-577), discuss multi-node spread-spectrum communication networks in which multiple orthogonal sequences within a relatively narrow bandwidth are assigned to each node, whereby a corresponding multiplicity of information bits can be simultaneously transmitted and/or received by each node--thereby providing a correspondingly higher data rate. A specified segment of each sequence available to a node of the network is designated as a "symbol". In the case of a repetitive sequence, a symbol could be a complete period of the sequence. The time interval during which a node transmits or receives such a symbol is called a "symbol interval". In a multi-node spread-spectrum network employing multiple orthogonal sequences, all the nodes can simultaneously transmit and/or receive information-bearing symbols derived from some or all of the sequences available to the nodes.
The emphasis in the aforementioned Enge et al. and Pahlavan et al. papers is on network performance, especially in certain kinds of signal environments. Neither paper recommends or suggests using any particular set of mutually orthogonal spreading codes for generating multiple orthogonal sequences; and neither paper discloses how to derive or generate suitable mutually orthogonal spreading codes. However, methods of generating families of sequences that are pairwise "almost orthogonal" by using two-register sequence generators have been known for some time.
In a paper entitled "Optimal Binary Sequences for Spread-Spectrum Multiplexing" by R. Gold, (IEEE Transactions on Information Theory, Vol. IT-13, October 1967, pp. 119-121), so-called "Gold codes" were proposed for use as spreading codes in multi-node direct-sequence spread-spectrum communication networks of the CDMA type. A Gold code is a linear recursive sequence that is generated by a product f.sub.1 f.sub.2, where f.sub.1 and f.sub.2 comprise the members of a so-called "preferred pair" of primitive polynomials of the same degree n over a field GF(2). A primitive polynomial of degree n is defined as a polynomial that generates a maximal-length linear recursive sequence (MLLRS), which has a period of (2.sup.n -1). The required relationship between f.sub.1 and f.sub.2 that makes them a preferred pair is described in the aforementioned paper by R. Gold.
A Gold code is a particular kind of "composite code". Other kinds of composite codes include "symmetric codes" and "Kasami codes". A symmetric code is similar to a Gold code in being generated by a product f.sub.1 f.sub.2 of a pair of primitive polynomials, except that for a symmetric code the polynomial f.sub.2 is the "reverse" of primitive polynomial f.sub.1, i.e., f.sub.2 (x)=x.sup.n f.sub.1 (1/x), where n=deg f.sub.1 =deg f.sub.2. The correlation properties of Gold codes and symmetric codes are discussed in a paper entitled "Crosscorrelation Properties of Pseudorandom and Related Sequences" by D. V. Sarwate and M. B. Pursley, (Proceedings of the IEEE, Vol. 68 No. 5, May 1980, pp. 593-619). Kasami codes differ from Gold codes in that for Kasami codes, the polynomials f.sub.1 and f.sub.2 are not of the same degree. Kasami codes are also discussed in the aforementioned paper by M. B. Pursley and D. V. Sarwate. The concept of a "composite code" can be broadened to include sequences obtained from a two-register sequence generator, where the sequences generated in the two registers can be quite general.
Predominant among the reasons that have militated against using direct-sequence spreading codes for multi-node spread-spectrum communication networks of the prior art is the so-called "near-far" problem. If the nodes of a multi -node spread-spectrum communication network are widely distributed so that power levels for different nodes can differ markedly at a given receiver in the network, then at the given receiver the correlations of a reference sequence with a sequence that is transmitted by a nearby node are apt to be stronger than correlations of the reference sequence with a version of the reference sequence that has been transmitted from a greater distance. Adverse effects of the "near-far" problem can include periodic strong correlations in information-bit errors, and false synchronization. To avoid such adverse effects, frequency hopping has been preferred in the prior art for multi-node spread-spectrum communication networks--especially for tactical networks where the nodes are widely distributed. Until recently, most of the research funding and efforts in connection with multi-node spread-spectrum communication networks have been directed toward tactical networks, thereby virtually precluding significant research on direct-sequence spread-spectrum communication networks.
Hybrid frequency-hopped and direct-sequence spread-spectrum communication networks have been proposed for tactical applications. However, the frequency diversity provided by "hopping" of the carrier readily enables rejection of unintended signals, thereby making the choice of a particular spreading-code sequence relatively unimportant. Consequently, there has been substantially no research in the prior art on the use of Gold codes and other composite codes for hybrid frequency-hopped and direct-sequence spread-spectrum communication networks.
Direct-sequence spread-spectrum communication networks have received recent attention in connection with the development of wireless local area networks (LANs), personal communications networks (PCNs), and cellular telephone networks utilizing communications satellites. The "near-far" problem is ordinarily not an issue for LANs and PCNs, because the nodes in such networks are generally distributed at distances that are not very far from each other. For cellular telephones, the "near-far" problem is not an issue in satellite applications, because all transmitters in the "spot beam" from a satellite are roughly at the same distance from the satellite.
Several wireless LANs are described in an article entitled "Spread Spectrum Goes Commercial" by D. L Schilling, R. L Pickholtz and L. B. Milstein, IEEE Spectrum Vol. 27, No. 6, August 1990, pp. 40-45. For indoor spread-spectrum communication networks (e.g., wireless LANs), spectrum spreading has commonly been employed in "star network" configurations. In a star network, the nodes are normally synchronized with a master controller, so that each node of the network can use a different offset of the same spreading-code sequence. False synchronization is not ordinarily encountered with star networks. In circumstances in which two or more star networks, each utilizing a different spreading-code sequence, operate in close proximity to each other, composite codes could be used to advantage to prevent interference between neighboring star networks. However, in the prior art, reliance has usually been placed upon the distance between the individual star networks, and upon signal-attenuating structures (e.g., walls) separating the individual star networks, as well as upon cross-correlation properties that are expected of random uncorrelated spreading-code sequences, to enable one star network to reject signals from another star network in its vicinity. Consequently, composite codes have generally not been used in star networks.
In PCNs, the use of composite codes as spreading-code sequences has not yet received much attention, because factors such as size, weight and power considerations have generally favored simplicity over performance. Techniques involving satellite-based CDMA cellular radio networks have emerged from developments in wireless LANs, but have generally been concerned with coding and systems engineering rather than with spreading-code sequence generation.
To date, direct-sequence spectrum spreading techniques have been used primarily in applications requiring high multipath immunity, good time resolution, robustness, privacy and low probability of detection, and for which in-band interference and the "near/far" problem are manageable. Such applications have included satellite communications, star networks in office environments, mobile radio, and positioning and navigation applications. The use of composite codes (e.g., Gold codes or symmetric codes) for spectrum spreading in such applications has not heretofore been deemed appropriate, because composite codes would require significantly greater hardware complexity to implement than MLLRSs without seeming to provide sufficient compensating advantages over MLLRSs in terms of processing gain, the number of nodes that can be accommodated, the rate of data transmission, or robustness.